Image forming apparatuses have been marketed as electro-photocopying apparatuses, printers, facsimile devices, or the like, or as multifunctional apparatuses having at least one combination of these devices. Such image forming apparatus includes a fixing belt, an image carrying belt (e.g., intermediate transfer belt), and a transport belt for transporting a recording medium or a document.
Conventionally, belts in the above-mentioned image forming apparatus are provided with a belt straightener to prevent the belt to wave. Such a straightener protrudes from an inner circumferential portion of the belt and engages with a groove-type engagement portion provided to a belt roller around which the belt extends. With such arrangement, the belt straightener may not disengage from the groove-type engagement portion, thereby preventing the belt from waving.
Conventionally, belt rollers include solid-type rollers. However, fixing belt rollers are tubular in order to accommodate a heater inside and have a relatively small heat capacity so as to insure a short heat-response time. Furthermore, the use of tubular rollers for image carrying belts and transport belts is also desired so as to reduce the weight of the image forming apparatus. Furthermore, belt rollers disposed near a fixing unit are more susceptible to heat energy generated in the fixing unit. If a solid-type roller is used in such area, heat dissipation from the roller is more difficult. Accordingly, tubular rollers are preferred in order to reduce heat effects in such areas. In view of such background, tubular belt rollers are preferred.
However, tubular rollers have a drawback because their strength is reduced in the engagement portion area where the belt straightener is provided. The formation of the engagement portion (i.e., for example, in the form of a groove) in the tubular roller leads to a relatively smaller thickness at the groove portion, thus reducing the roller's strength at the groove portion. If a tubular roller having a relatively large thickness is used to lessen such drawback, the heat capacity of the roller increases, leading to a deterioration of the heat-response time of the tubular roller and lowering the fixability of toner images. Furthermore, the weight of a tubular roller having a relatively larger thickness increases, thereby lessening the desirable weight reduction effect obtained by using the tubular roller.
In order to increase the roller's strength, the depth of the engagement portion can be made shallower. However, a shallower engagement portion may not adequately prevent the belt from waving because belt waving is prevented by the engagement of the belt straightener with an engagement portion having an appropriate depth.